In semiconductor integrated circuit (IC) fabrication, devices such as component transistors are formed on a semiconductor substrate. During the fabrication process, various materials are deposited or etched on different layers in order to build or form the desired integrated circuit. The various layers define devices that are interconnected by metallization lines. During certain plasma enhanced processes performed upon substrates that already contain devices and metalized lines, a substantial amount of charge may accumulate on the surface of the substrate. This charge accumulation may not be uniform across the substrate may cause destructive currents to be induced the metalized materials and/or cause arcing within or across dielectric layers. The currents and/or arcing may destroy or damage devices that have previously been formed on the substrate. To mitigate the charging effects and avoid charging damage, the power supplied to a plasma within a plasma enhanced reactor may be pulsed during all or part of the plasma process.
When pulsing the plasma, the power from a radio frequency (RF) power source may be coupled through a dynamically tuned matching network (also referred to as a match unit) to an antenna or electrode within the reactor. The pulsed power is coupled from the antenna or electrode to process gases within the reactor to form a plasma that is used for the etching process. The matching network ensures that the output of the RF source is efficiently coupled to the plasma to maximize the amount of energy coupled to the plasma (e.g., referred to as tuning the RF power delivery). Thus, the matching network ensures that the total impedance (i.e., plasma impedance+chamber impedance+matching network impedance) is the same as the output impedance of the RF power delivery.
In continuous wave and single level pulsing (e.g., pulsing between an on state and an off state), there is only one state to tune to (e.g., the on state). However, in dual level pulsing, the RF power source is switched between a high state and a low state (e.g., not an off state), at a pulse frequency and a pulse duty cycle. Thus, the RF power delivered alternates between two different on states that have to be tuned to. However, due to the high frequency at which the different on states are provided during dual level pulsing processes, tuning with variable capacitors is not possible due to the low speed of variable capacitor movements.
In view of the foregoing, the inventors have provided embodiments of improved methods and apparatus for frequency tuning in process chambers using dual level pulsed RF power.